Ring binder mechanisms having rings for selectively retaining loose-leaf pages are well known. These mechanisms are commonly fastened to other structures such as notebook covers, files, clipboards, and the like to enable the rings to retain loose-leaf pages. The rings of ring binder mechanisms typically include two ring members that are selectively movable between an open position for receiving loose-leaf pages and a closed position for retaining loose- leaf pages. Conventionally, the ring members are hingedly connected to a base for pivoting movement relative the base. When the ring binder mechanism retains loose-leaf pages in the closed position, it is preferable for the rings to remain secured in the closed position. For certain types of ring binder mechanisms, when the rings are in the closed position, the rings are biased by a spring force towards the closed position to prevent unintended opening of the rings. The ring members of these and other types of ring binder mechanisms can also include interlocking formations that secure the ring in the closed position. However, known interlocking formations can be prone to disengagement in response to forces urging the ring members to pivot toward their open position. For example, if a ring binder loaded with paper is dropped, the ring members can be subject to lateral forces in the same direction as the ring member naturally opens. Some interlocking formations may be inadequate to resist such forces.